Hybid solar and fuel fired electrical generating system

ABSTRACT

An electric power generation system combines a gas turbine generator with a solar power plant and utilizes the gas turbine exhaust for steam superheating and feed water heating only. The solar heater is only utilized for boiling or evaporation of feed water into steam, the feed water having previously been heated by a downstream portion of the turbine exhaust. In order to balance the disparity between the specific heats of water and steam to thus optimize the system, the steam is superheated by an upstream portion of the turbine exhaust to first drive a high pressure steam turbine and then reheated by the same exhaust over the same temperature range to drive a low pressure steam turbine.

The present invention is directed to a hybrid solar and fuel firedelectrical generating system and more specifically, to where the fuelportion of the power plant is a gas turbine where in addition togenerating electricity the hot exhaust gas of the turbine is used forproducing steam in combination with the solar unit for driving steamturbine generators.

BACKGROUND OF THE INVENTION

Combined cycle electrical generating systems using solar and gas turbineunits are probably known as illustrated in U.S. Pat. No. 5,444,972. Inaddition it is believed that Bechtel Corporation of San Francisco,Calif. have designs that have added to a standard General Electric gasturbine power plant (which by the way also used high pressure and lowpressure steam turbines), a solar evaporator. However, in both of theabove installations, there was no specific effort to optimize theoverall system. Rather the solar energy portion of the system was merelyadded to the combined cycle, utilizing the original gas turbine steamgeneration equipment and cycle layout as originally designed for fuelfiring.

OBJECT AND SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an improvedhybrid solar and fuel fired electrical generating system.

In accordance with the above object there is provided an electric powergeneration system having a substantially closed feed water/steam path toprovide a common mass flow comprising a gas turbine generator having ahot exhaust gas stream. First heat exchanger means located in adownstream portion of the hot exhaust gas heats the feed water tosubstantially its evaporation temperature. Solar boiler means connectedto the first heat exchanger evaporates the feed water. A high pressuresteam turbine generator and a low pressure steam turbine generator,having a low pressure exhaust, are connected to a condenser whichthereby supplies the feedwater. A second heat exchanger means at leastpartially located in an upstream portion of the hot exhaust gas of theturbine receives evaporated feed water from the solar boiler means andalso the low pressure exhaust of the low pressure steam turbine andsuperheats it to a predetermined temperature for driving both said highpressure and low pressure steam turbines. The absolute heat energy perdegree of temperature rise supplied by the second heat exchanger meansfor superheating is substantially equal to the heat energy per degree oftemperature rise provided by the first heat exchanger means to heat saidfeed water to said evaporation temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of the total system.

FIG. 2 is a graph helping to explain the concept of the invention.

FIG. 3 is a characteristic curve illustrating the actual operation ofthe invention.

FIGS. 4 and 5 are curves similar to FIG. 3 illustrating theoreticalnondesired operational modes.

FIG. 6 is a temperature-entropy diagram illustrating the presentinvention.

FIG. 7 is a schematic representation which is an alternative to FIG. 1.

FIG. 8 is the diagram of FIG. 6 modified to illustrate FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 illustrates the electrical power generation system of the presentinvention which has as its key components a fuel powered gas turbinedriving a generator 12 and a solar boiler 13. As will become apparentthe solar heat used in conjunction with the gas turbines exhaust gasheat, the gas stream being shown as the dashed line 14, will make muchmore steam power than can be made by using each separately, or in acombined or hybrid system as discussed above, which is not optimized. Ingeneral the present invention realizes that since the gas turbineexhaust heat has the characteristic of giving up one percent of its heatwith a drop of one percent of its temperature from its given datum, itshould not be used for boiling, which occurs at a constant temperature,but only for feed water heating to the evaporation temperature and alsosteam superheating. On the other hand, since solar heating occurs atclose to a constant temperature it is best mainly used for boiling orbringing the feed water to its evaporation temperature. Thus asillustrated in FIG. 1, the solar boiler could as well be a nuclearboiler since that has the same type of characteristic; that is, aconstant temperature even though heat is being removed from the systemas contrasted with the gas turbine exhaust gas.

Referring now to the hardware implementation of the system of FIG. 1,the exhaust gas 14 of the gas turbine 11 first is routed to a highpressure super heater 16 and a low pressure reheater. As indicated theseare actually interleaved heaters 18 in the form of tube sheets. Highpressure super heater 16 superheats the high pressure steam generated bythe solar boiler 13.

Thus the solar boiler temperature is near the evaporation temperatureand super heater 16 heats the resultant steam up to the maximum approachtemperature used to drive the high pressure steam turbine generator 21.

After expansion in the high pressure .steam turbine 21 the exit line 22is still superheated at the original exit temperature of solar boiler13. The low pressure reheater 17 again reheats the steam to the maximumapproach temperature and drives the low pressure steam turbine generator23. The reheated steam is then expanded through the low pressure steamturbine to the condenser unit 24 and then after condensing the water ispumped by pump 26 to feed water heater (or heat exchanger) 27. This islocated in the downstream portion 28 of the gas turbine exhaust 14. Withthe use of this exhaust the feed water is again heated to up to near itsboiling point and fed to the solar boiler 13 to complete the closed feedwater/steam path. Since this is a closed path, of course, there is acommon mass flow through the system. The super heater and reheater 16,17 are located in the upstream or hotter portion of the hot exhaust gasstream 14.

Theoretically when using the exhaust gas heat of the gas turbine solelyfor water heating and super heating, it is desirable for the bestthermodynamics to have close to a constant temperature differencebetween the turbine gas flow as it cools and the countercurrent waterand steam flows as they heat up by taking heat from the gas flow.However, FIG. 2, which is a temperature-entropy (H) characteristicindicates an inherent problem. The exhaust gas has a specific heat (Cp)of about 0.25. On the other hand steam has a specific heat of about 0.5and water of 1.00. This leads to an imbalance as the water has aspecific heat of about twice the specific heat of the steam. Thus theamount of water flow heat pick up degree which would match the gasflow's heat pickup degree is only about half the amount of steam flowwhose heat pickup per degree would match the gas flow's heat give up perdegree.

As stated above the ideal optimum thermodynamic situation is where thereis a constant temperature in the heat exchange between the turbine gasflows and the steam and water. This is illustrated in FIG. 3 where thesteam, water, and gas curves are shown with the horizontal axis beingheat exchange in BTUs per hour and the vertical axis being in ° F.temperature. Specific temperature values are indicated for one exampleto be discussed below. The ideal characteristics of FIG. 3 can berealized as will be discussed in detail below by heating each pound ofsteam twice over the same temperature range. However, if this is notdone, the curves of FIGS. 4 or 5 show theoretical undesired results.These curves are conceptual only and simplified; thus they do not matcha real situation. Thus in FIG. 4 the water and gas counterflows arematched leaving a mismatch with the steam; in FIG. 5 the steam ismatched leaving a mismatch with the water.

The temperature-entropy characteristic of FIG. 6 succinctly illustratesthe technique of the present invention which achieves the idealcharacteristics of FIG. 3. The main curve 31 of FIG. 6 is a standardtemperature-entropy curve where in the interior of the bell shaped curveis the wet region, on the left side is liquid and on the right is thedry region or superheat and vapor region.

Now relating the system of FIG. 1 to the chart of FIG. 6, the feed waterheater 27 is illustrated by the curve 32 which heats the feed water upto its boiling point which is indicated at 33 and, for the example ofthe present invention is approximately 566° F. at a pressure of 1190psia. The cross hatched portion A₁ under the curve up to 33 and goingdown to absolute zero temperature is the heat energy supplied by thatstage. Then the horizontal line 35 is the latest heat of evaporationwhich occurs in the solar boiler 13 to change the phase of the waterfrom liquid to vapor which occurs at 36. On the other side of the bellcurve 31 the dashed line shown at 37 are isopressure lines at 5 psi, a100 psia and 1000 psia. Substantially along the 1000 psi line the solidline 38 shows the operation of super heater 16 which superheats thesteam up to the temperature indicated of 1050° F. at a pressuresubstantially similar to the original pressure 1130 psia. Then on line22, this is dropped by typical valve means on the high pressure steamturbine and by the turbine action, per se, to the low pressure of 150psia and 566°, the original evaporation temperature. The low pressuresteam is again reheated by the low pressure reheater 17 as shown by thesolid line 39 back to 1050° at substantially a pressure of 150 psi. Andthen the action of the low pressure turbine in conjunction withcondenser 24 (see line 41) drops the exhaust down to substantially 1psia at about 100° F. temperature. It is critical that this line 41indicating the exhaust of the low pressure turbine 23 hits the curve 31at the location indicated so that the temperature and pressurecombination provides an exhaust which is not totally dry and not toowet.

Thus referring to the crosshatched portions, the heat added by the highpressure superheater 16 is shown by the area A₂ and the heat added bythe low pressure reheater 17 by the area A₃. As shown by the equation itis desirable for optimum efficiency that area A₁ divided by thetemperature rise, T₂ -T₁ equal to the sum of A₂ and A₃ divided by thetemperature rise, T₃ -T₂. When this is done it will effectivelycompensate for the difference between the specific heats of water andsteam as discussed above to thus yield the idealized characteristiccurves of FIG. 3.

To explain the foregoing by equation, the following three apply:

    (1) M*Cp.sub.w =M*Cp.sub.g

    (2) ΣM*Cr.sub.s =M*Cp.sub.g

    (3) ΣM*Cp.sub.s =M*Cp.sub.w

where

M=Mass flow

Cp=Average specific heat

Equation (1) shows the matching of the product of mass flow and specificheat of water with gas flow, and equation (2) the matching of thesuperheating steam to gas. Finally equation (3) is the necessarycondition for equations 1 and 2 to hold. Since the specific heats of gasand water are not equal, (one is double the other) by superheating thesteam twice, first at high pressure and then at low pressure equation(3) is effectively satisfied.

Thus, in general the following are design criteria: 1) steam is reheatedtwice compared to water; 2) the same temperature range is used for boththe high pressure superheater 16 and the low pressure reheater 17; 3)the temperature of the solar boiler, given in the example of 566°, ischosen to provide an inexpensive solar boiler of the trough type (alsothe pressure of 1190 psia is chosen for good compatibility with thesolar boiler output); 4) the low pressure turbine drop to the condenseris neither dry or too wet, 5) the inlet pressure to the high pressureturbine 21, for example, 1130 psia, is not too high for a commercialturbine; 6) the pressures of heaters 16 and 17 are chosen to provide thedesired equality with feedwater heating.

A theoretical system has been designed and the attached Table Iillustrates the operating parameters.

As illustrated, for example, by the Table 1, the embodiment of FIG. 1with the proper temperatures and pressures provides optimum efficiency.For example, in some installation a pressure of 2150 psia might providesuperior efficiencies. However FIG. 7 illustrates an alternative whichis a modification of FIG. 1 which uses an additional low pressure solarboiler 41 with a modified feedwater arrangement. This includes theexisting feedwater heater which is now divided into low temperature/highpressure and high temperature/high pressure sections 40 and 42 and a lowtemperature/low pressure section 43. Sections 40 and 43 are fed via thehigh pressure pump 44 and low pressure pump 46, respectively, whichreceive feedwater from condenser 24. The high temperature section 42again heats the feedwater to its evaporation temperature and thenallowing the high pressure solar boiler 13 to change it to vapor. Thenthe output of boiler 13 goes to the high pressure superheater 16 asbefore. However in accordance with this modification the lowtemperature/low pressure section 43 now feeds the new low pressure solarboiler 41 which heats the feedwater to its evaporation point and then anadditional new low temperature/low pressure superheater section 47superheats the steam and couples it via line 48 to the input line 22 ofthe low pressure reheater 17.

The feedwater heating units 42 and 40 are sequentially in the lowtemperature portion of the exhaust stream 14 from gas turbine 11 as aresuperheater 47 and low temperature heater 43. Thus in effect two newheat supply sources have been provided as aptly illustrated by thetemperature-entropy diagram of FIG. 8. Here the heat energy supplied toarea A4 raising the feedwater to its evaporation temperature is thatsupplied by low pressure/low temperature heater 43 and then the lowpressure solar boiler 41 supplies the latent heat of evaporation whichthen is fed to the superheater 47 and as illustrated by the line 48merges with the isopressure line 39 which is actually the input 22 tolow pressure heater 17 as illustrated in FIG. 7. The degree temperaturerise supplied by superheater 47 is A'₁. Referring to the equation ofFIG. 6 this would be respectively added to the left of the equation withan adjustment for mass flow. In this embodiment the balance of theequation occurs at the high pressure boiling point, also known as the"pinch" temperature. Referring to FIG. 6 this is about 566° F. Althoughit is believed that the technique of FIGS. 7 and 8 may be lessefficient, it is illustrative as to how the concept of the invention inbalancing the heating perunit temperature rise of the feedwater with thesuperheating can be accomplished in many different ways. Thus animproved hybrid solar and fuel fired electrical generating system hasbeen provided.

                  TABLE 1                                                         ______________________________________                                        Superheater 16 Inlet Steam Temperature (F.)                                                             566                                                 Superheater 16 Outlet Steam Temperature (F.)                                                            1050                                                Superheater 16 Inlet Gas Temperature (F.)                                                               1109                                                Superheater 16 Outlet Gas Temperature (F.)                                                              591                                                 Reheater 17 Inlet Steam Temperature (F.)                                                                567                                                 Reheater 17 Outlet Steam Temperature (F.)                                                               1050                                                Reheater 17 Inlet Gas Temperature (F.)                                                                  1109                                                Reheater 17 Outlet Gas Temperature (F.)                                                                 591                                                 Feedwater Heater 27 Inlet Water Temperature (F.)                                                        104                                                 Feedwater Heater 27 Outlet Water Temperature (F.)                                                       554                                                 Feedwater Heater 27 Gas Inlet Temperature (F.)                                                          591                                                 Feedwater Heater 27 Gas Outlet Temperature (F.)                                                         150                                                 Solar Boiler 13 Duty (Btu/hr)                                                                           2.30E + 0.8                                         Solar Boiler 13 Steam Production (lb/hr)                                                                3.65E + 0.5                                         Total Net Power (MW)      141.74                                              GT 11 Power (MW)          70.95                                               ST 21,23 Power (MW)       70.8                                                GT 11 Heat Input (Btu/hr) 6.84E + 0.8                                         Plant Heat Rate (Btu/hr)  4828                                                Plant Efficiency (%)      70.7                                                Adjusted Heat Rate (Btu/hr)                                                                             6447.7                                              Standard Steam Turbine Power                                                                            108.3                                               Steam Turbine Outlet Quality                                                                            0.998                                               Solar Boiler Pressure (Psia)                                                                            1190                                                HP Steam Turbine Inlet Pressure (Psia)                                                                  1130.5                                              HP Steam Turbine Reheat Pressure (Psia)                                                                 159                                                 LP Steam Turbine Inlet Pressure (Psia)                                                                  150.4                                               Steam Turbine Condenser Pressure (Psia)                                                                 1                                                   ______________________________________                                    

What is claimed is:
 1. An electric power generation system having asubstantially closed feed water/steam path to provide a common mass flowcomprising:a gas turbine generator having a hot exhaust gas stream; alow temperature heat exchanger located in a downstream portion of saidhot exhaust gas for heating said feed water to substantially itsevaporation temperature; a solar boiler connected to said lowtemperature heat exchanger for evaporating said feed water; a highpressure steam turbine generator and a low pressure steam turbinegenerator; first and second high temperature heat exchangers located inan upstream portion of said hot exhaust gas of said turbine, said firsthigh temperature heat exchanger receiving evaporated feed water fromsaid solar boiler and superheating it to a predetermined temperature fordriving said high pressure steam turbine, such high pressure steamturbine having a low pressure exhaust at a temperature near saidevaporation temperature, said second high temperature heat exchangerreceiving such exhaust from said high pressure turbine, and reheating itto substantially said same predetermined temperature for driving saidlow pressure steam turbine, the exhaust of said low pressure steamturbine being connected to a condenser, where steam is changed to water,and then by a high pressure feedwater pump to said low temperature heatexchanger; the absolute heat energy per degree of temperature risesupplied by said first and second high temperature heat exchangers andutilized by said high pressure and low pressure steam turbines beingsubstantially equal to the heat energy provided by the feed water heatexchanger to heat said feed water to said evaporation temperature.
 2. Asystem as in claim 1 where said exhaust of said low temperature steamturbine is at substantially or below ambient air pressure but is at atemperature so that the exhaust is not totally dry or too wet.
 3. Asystem as in claim 1 where the pressures of said first and second hightemperature heat exchangers are chosen to provide said equality of heatenergy input per degree of temperature rise.
 4. A system as in claim 1where said solar boiler provides substantially all of the heat necessaryfor changing the state of said feed water from liquid to vapor.